1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, copier, facsimile machine, or the like, and relates particularly to the correction of density and color misregistration of toner images in the image forming apparatus.
2. Description of the Related Art
Recently, following the rapid shift from monochrome inkjet printers to color inkjet printers, electrophotographic image forming apparatuses (copiers, printers) also have been shifting from monochrome machines to color machines. Some of the color machines are of the tandem type that forms a monochromatic toner image for each color on respective drums and successively transfers the monochromatic toner images onto an image carrier, thereby recording a color image. Meanwhile, some of the color machines are of the single drum type that repeats the process of forming a monochromatic toner image on a drum and transferring each monochromatic image onto an image carrier the same number of times as there are colors, thereby recording a color image. Although the tandem type is inferior to the single drum type in terms of size reduction and cost, the tandem type can form images of respective colors independently (perform image formation in a single pass) and is therefore suitable for faster operation. Accordingly, the tandem color image forming apparatuses, which can achieve an image forming speed equivalent to that of the monochrome machines, have recently been receiving significant attention.
When toner images are successively transferred onto the image carrier in the tandem type, some apparatuses use a method of temporarily forming a color image on an intermediate transfer belt serving as an image carrier and then transferring the color image onto a recording medium serving as an image carrier. In this method, in order to perform density correction or color misregistration correction, a pattern for correction is formed on the intermediate transfer belt and detected by a patch detection sensor. Conventionally, however, when correction is performed by this method, it is difficult to increase the detection accuracy because the output of the patch detection sensor that has detected the correction patch varies under the influence of unevenness of drive of a driving motor that rotates the intermediate transfer belt or a gear, flutter of the intermediate transfer belt, or the like. In order to prevent the flutter of the intermediate transfer belt, it has been suggested that a backup roller be provided on the reverse side of a portion detected by the patch detection sensor, as shown in FIG. 12. Furthermore, as shown in FIG. 13, some apparatuses have a configuration in which the patch detection sensor is disposed in a position facing, for example, the driving roller of the intermediate transfer belt so as to reduce the influence of flutter of the intermediate transfer belt. Moreover, a technique of forming the patch detection sensor and a plate for calibration from the same material and thus increasing the positional accuracy of the patch detection sensor and the calibration plate has been proposed (Japanese Patent Laid-Open No. 11-237773).
In the conventional technique of providing the backup roller, there is the risk that the lifetime of the intermediate transfer belt will be affected. In the case where the patch detection sensor is disposed in a position facing the driving roller of the intermediate transfer belt, since the diameter of the roller also has been decreased with reduction in the size of the apparatuses, a slight misalignment will cause the reflection light to be eclipsed, and consequently a sufficient output cannot be obtained. Moreover, the need for fine adjustment during assembly of the apparatuses causes an increase in the assembly cost and also requires time for adjustment. Reflective optical sensors have a characteristic of being sensitive to angular displacement, as shown in FIG. 14, and thus it is undesirable to dispose the sensor in a position facing the roller, which has a curved surface. Moreover, the technique of Japanese Patent Laid-Open No. 11-237773 is ineffective when a patch on the intermediate transfer belt is read.